Rav Nosson Zvi Finkel, the Alter of Slabodka

A boy once came with a sullen face.  R’Nosson Zvi began to whistle and sing, “Should a person blessed with delights be sad” again and again until the boy was affected by the tune and the boy cheered up.

From Sparks of Mussar by R Chaim Ephraim Zaitchik